1. Field of the invention
This invention relates to a heat-sensitive recording material, more specifically to a heat-sensitive recording material containing a heat-sensitive recording component coloring characteristics such as a coloring temperature, etc. of which are controlled.
2. Prior art
A heat-sensitive material generally comprises a support and a heat-sensitive recording layer mainly comprising an electron-donative dye precursor generally colorless or pale colored and an electron-accepting developer as main component provided on the support. By heating the heat-sensitive material with a thermal head (heat head), a thermal pen, and a laser beam, etc., the dye precursor and the developer are immediately reacted to obtain a colored image. This is disclosed in, for example, Japanese Patent Publications No. 4160/1968 and No. 14039/1970, etc.
Recording by using such a heat-sensitive recording material can be carried out with a relatively simple device, and the device has advantages that maintenance is easy, no noise generates, etc. Thus, it is utilized in various fields such as an instrumental recorder, a facsimile machine, a printer, terminals of a computer, labeling, a ticket vending machine, etc.
On the other hand, in many uses of heat-sensitive recording materials, it has been desired to develop a multi-colored heat-sensitive recording material which can form two or more color tones by the difference of heating temperature. As a method for realizing such a multi-colored heat-sensitive recording material, the following methods have been known. One of them is a method in which color-forming temperatures are controlled by changing melting points of compounds constituting heat-sensitive recording components, and at low temperature heating, only a heat-sensitive recording component having a low color-forming temperature forms a color, while at a high temperature, the heat-sensitive recording component having a low color-forming temperature and a heat-sensitive recording component having a high color-forming temperature simultaneously produce colors to form a different color tone from that of the low-temperature heating as disclosed in Japanese Patent Publication No. 69/1974. As another method, several kinds of heat-sensitive recording components are contained in respective layers laminated on a support separately, and a heat-sensitive recording component contained in a surface layer which is close to a heat source is color-formed at a lower heating temperature, and a heat-sensitive recording component contained in a layer which is farthest from the heat source is color-formed at a higher heating temperature as disclosed in Japanese Patent Publication No. 27708/1974.
In the former method in which a color-forming temperature is controlled by changing a melting point of the compound constituting the heat-sensitive recording component, there are problems that raw materials which can be used in the method are limited and a balance with the other characteristics cannot sufficiently be obtained. In addition, to contain the heat-sensitive color-forming components different in a formed color tone in the same layer, it is necessary to prevent color formation by an interaction of the different kinds of heat-sensitive recording components to each other. Also, in the latter method in which respective kinds of heat-sensitive recording components are contained in a separate layer among the laminated layers, there is a problem that the layer constitution of the multi-colored heat-sensitive recording material becomes complex whereby productivity becomes worse, or the like.
Moreover, in the multi-colored heat-sensitive recording material, it is important that two kinds or more of color tones are formed within a narrow temperature range (a printing energy range), i.e., formed color tones are clearly separated to each other. To realize the above, it is required that, in a color-forming behavior of a high-temperature color-forming component, the difference between the highest temperature (the maximum energy) of a heating temperature (an applied energy) at which no color is formed and the lowest temperature (the minimum energy) at which color formation reaches saturation is small, that is, start of color-formation relative to the temperature (energy) is steep. By making start or raise of color formation of the high-temperature color-forming component steep, within a narrow temperature range (a printing energy range), the high-temperature color-forming component does not form a color at low-temperature heating but suddenly forms a color only at a certain temperature or more so that a color tone by low-temperature color-forming is sharp and color separation becomes clear.
However, in the above-mentioned conventional multi-colored heat-sensitive recording materials, start of color formation of the high-temperature color-forming component is generally gentle and color separation is unclear. Also, even if a color formation temperature of the high-temperature color-forming component is shifted to a higher temperature side by a certain means, whereas low-temperature color formation becomes sharp and color separation becomes clear with a certain extent, high energy is required for color-formation of the high-temperature color-forming component whereby it cannot be practically used. To solve these problems, it has been proposed a method of controlling color-forming characteristics such as a color-formation temperature, etc. by incorporating a compound constituting the heat-sensitive recording components into a microcapsule to have a role of a color-formation controlling layer to the wall of the microcapsule.
For example, in Japanese Provisional Patent Publication No. 282115/1996, it has been proposed a method in which a plural number of electron-donative dye precursors and electron-accepting compounds having different color tone to be color-formed are contained in the same layer, and at least one of said electron-donative dye precursors is contained in a microcapsule. According to this method, a color-forming temperature can be heightened by reacting the electron-donative dye precursor and the electron-accepting compound. Also, the electron-donative dye precursor incorporated into the microcapsule and the electron-donative dye precursor not incorporated therein are not interacted to each other so that two kinds or more of color tones to be color-formed can be obtained by one layer of the heat-sensitive recording layer.
Also, by incorporating a compound constituting the heat-sensitive recording component, e.g., an electron-donative dye precursor, in a microcapsule, in a heat-sensitive recording material having a heat-sensitive recording layer containing the microcapsule, control of color-forming characteristics such as a color-forming temperature can be realized. Moreover, even if an electron-donative dye precursor and an electron-accepting compound are used in combination, which cause background surface fogging when they are used in combination by not separating with each other, by separating the electron-donative dye precursor and the electron-accepting compound, a heat-sensitive recording material having good color-forming can be obtained without causing background fogging.
Moreover, even if a substance which color-forms a heat-sensitive recording material such as an organic solvent is attached to the heat-sensitive recording material, such an erroneous color-formation of the heat-sensitive recording material can be prevented by providing a characteristic of not dissolving a color-forming substance by an organic solvent, etc. to a color-formation controlling layer.
As a method for forming a color-formation controlling layer on the surface of the compound constituting the heat-sensitive recording components, various kinds of methods for making microcapsule can be applied to. Specific examples thereof may include the interfacial polymerization method, the coacervation method, the spray drying method, the emulsion evaporating solidification method, the emulsion cooling solidification method, etc. However, the compounds constituting the heat-sensitive recording components are usually solid materials. Thus, when the interfacial polymerization method is to be employed, there is a method in which the compounds constituting the heat-sensitive recording components are dissolved in a solvent, starting materials of a capsule wall are added thereto, the mixture is dispersed in a medium such as water, etc., and then, the solvent dissolving the color-forming component is evaporated and simultaneously the capsule membrane is polymerized. However, there are drawbacks that procedure is complicated and productivity is lowered. Also, when the coacervation method is employed, there are drawbacks that preparation conditions of microcapsules are extremely limited and characteristics of the formed capsule wall are also limited. When the spray drying method is employed, there are drawbacks that uniformity of the capsule membrane is poor, a particle size of the capsule likely becomes too large, and printing property when applied to a heat-sensitive recording material becomes bad. When the solution cure coating method is employed, there is a drawback that a particle size of the capsule likely becomes too large, etc. When the melt dispersion cooling method is employed, there are drawbacks that a melting point of the capsule membrane is limited to a relatively low melting point so that the characteristics of the formed capsule wall are also limited.
On the other hand, in Japanese Provisional Patent Publication No. 142025/1997 which corresponds to U.S. Pat. No. 5,804,528, it is proposed a method for obtaining a multi-colored heat-sensitive recording material by using an electron-donative dye precursor as a compound constituting the heat-sensitive recording component, controlling color-formation characteristics by making an electron-donative dye precursor as a complex particle with a polyurea or a polyurethane, and by adding said complex particles to a heat-sensitive recording material to obtain a multi-colored heat-sensitive recording material. According to this method, it is disclosed that lowering in sensitivity is little and color-formation due to pressure or friction is hardly occurred as compared with the method of utilizing the usual microcapsule.
However, according to this method, a large amount of the polyurea or the polyurethane is to be contained in order to coat completely the electron-donative dye precursor so that a sufficient sensitivity cannot be obtained. Also, there are drawbacks that a range capable of controlling color-formation characteristics is limited with a certain extent, and start of color-formation likely becomes gentle.